There is a continuous demand for increasing the capacity of cellular phone base stations, as the number of users in each cell site grows. Wireless Service Providers who install the hardware systems and set up base stations are faced with challenging engineering and design trade-offs.
For example, an issue facing a provider when planning to install a new base station is the number of antenna and feeder lines required for the cell site. Typically, each base station has three sectors, each of which containing any number of antennas and feeder lines. Each antenna transmits and receives signals over a designated number of communication channels, based on the number of phone calls that will take place in each cell site. The number of necessary communication channels dictates the number of required transciver units (TRUs). However, it is often difficult to predict the number of necessary channels in a given location. Frequently, base stations are equipped with a limited number of antennas and feeder lines, based on budget constraints and uncertainty of future use. The number of antennas and feeder lines are further restricted by tower loading limitations, local zoning regulations and aesthetic concerns.
Once a base station has begun operation, the demand for simultaneous phone calls may increase over time. As such, the base station would need to transmit and receive signals over additional communication channels. To this end, operators have to employ additional TRUs with additional feeder lines and antennas, which translates to expensive capital expenditure. As pointed out earlier, the addition of feeder lines and antennas at many sites can be restricted due to tower loading, zoning, and aesthetic concerns.
One approach in prior art systems for responding to a growth in cellular call traffic is the use of additional transmitters coupled to corresponding combiners to allow the existing antenna and feeder lines to be shared by the two or more transmitters. Typically, the output port of the existing transmitter is coupled to an input port of a 2-way combiner, and the output port of the additional transmitter is coupled to the second input port of the combiner. The combiner then provides a combined output signal to the antenna of the base station. However, this approach causes a power drop of approximately 3 dB. Combiners with lower loss are available, but their advantage is offset by their higher cost.
Another approach in prior art systems for responding to call traffic growth is the use of multi-carrier power amplifiers (MCPAs), which include a linear amplifier to transmit more than one carrier radio frequency on a single line. However, MCPAs are relatively expensive. Furthermore, multi-carrier power amplifiers are typically arranged to first drop the power level of the signals received from the transceiver units (TRUs) to very low levels, and then amplify the signals again to the proper power levels. As such, the MCPA does not benefit from the existing power that is being generated by original base station equipment. Moreover, MCPA do not benefit from a power drop in the signal provided by the TRU unit. Hence the MCPA would continue to operate, despite the TRU's failure to provide an effective signal, resulting in power waste and consequent higher operating costs.
Thus there is a need for an efficient system that allows the base station to expand dynamically in response to increasing cell phone call traffic, without adding expensive equipment.